Optical lithography has been the workhorse of semiconductor fabrication since the inception of integrated circuits. The lensless proximity printing system gradually gave way to projection-printing systems, and one-to-one replication systems became reduction systems. It took this latest form from the 0.15-NA 436-nm g-line lens, featuring resolution over 2 μm with a 0.8 k1 factor, all of the way through raising the NA until the lens became too expensive to build at that time, reducing wavelength to reposition the NA for the next round of increases, and lowering k1 whenever the pace of NA and wavelength changes are behind the circuit shrinking roadmap, as shown in Fig. 8.1.
Outlook and Successors to Optical Lithography